Diverging Effects of NaCl and CsCl on the Mechanical Properties of Nanoconfined Water

Using an ultra-sensitive home-built atomic force microscope, we have studied the dynamic mechanical responses of pure water, 1 M NaCl, and 1 M CsCl aqueous solutions to understand the effects of ions on the viscoelastic properties of nanoconfined (≤ 1 nm) water films. In 1 M NaCl, we observed peaks in the Maxwell relaxation time, indicating a solid-like, elastic response due to jamming (dynamic solidification) during squeeze-out. NaCl also extended the range of ordering of water molecules further away from the mica surface up to 4–5 molecular layers (∼1 nm). By contrast, in 1 M CsCl, the relaxation time peaks were suppressed, even at high compression speeds, indicating a more liquid-like, viscous response. The addition of NaCl significantly increases the probability of the nanoconfined water film to react elastically in response to compression, while 1 M CsCl decreases the probability of water layers to show an elastic response. Our measurements support the notion that Na +  acts as a kosmotrope (order enhancer) and Cs +  as a chaotrope (order destroyer), directly influencing the hydration structure of water, and altering the mechanical response of the nanoconfined liquid layers to compression and squeeze-out.